returning spell

[Memento]

HI
I have a question regarding the spell returning.
I am often playing LA Marignon cranking a lot of soul contracts. To prevent the losing contract i often put them on my royal navigators and script them to cast returning. Now, if navigator got a lucky pick of S2 everything is ok - he uses his 2 astral pearls and get back to capitol (i am hoarding them in non capitol castle to make returning work).
But when the royal navigator has only 1S nothing seems to work. I am trying to give them 3 astral pearls so that he use 1 pearl to improve his S to 2 and other 2 pearls to actually cast returning - but this somehow does not work.
Does everybody know how to make spellcasting AI to cast returning on 1S mage?

thanks in advance